1. Field of the Invention
The present invention relates generally to a guidewire for directing a medical device to a precise location within a body lumen, such as blood vessels. Methods are provided for properly matching a guidewire and catheter together for safe operation and precise location. In particular, the present invention relates to apparatus and methods for guided atherectomy.
2. Description of the Background Art
Medical guidewires are used primarily to facilitate the placement of catheters and endoscopic instruments within the tortuous paths of body conduits. For example, if it is desirable to place a catheter within the vascular system of a patient, a guidewire is first inserted into the vessel and then guided through the tortuous path desired for the catheter. Then the catheter is threaded over the guidewire. As the catheter is advanced it tends to follow the direction of the guidewire so that it ultimately negotiates the same tortuous path. Once the catheter is in its final operative position, the guidewire can be removed leaving the catheter to perform its desired function.
Guidewires are traditionally utilized to negotiate the complex vascular system of a patient to guide a medical device, (e.g. a catheter) to a desired location. It has been in the past of paramount importance for the guidewire to have a shape which provides for superior navigation a patient""s vascular system. Inventions in the field include guidewires with floppy tips, improved methods of manufacturing, increased torquability and improved friction reducing features to help catheters move over the guidewires. Thus the focus of the prior art has been to create a guidewire with the ability to create a path along which a catheter could follow to reach a particular site of the body.
Guidewires often use transition areas of changing diameter along their length. A smooth transition gives the guidewire the ability to better negotiate tight bends in the anatomy of the patient. The transition area of a guidewire may be long or short, that is the change from one diameter along the length of the guidewire may occur over a few millimeters, or several centimeters. In the past the use of transition areas has been combined with the use of a filament wire which covers the narrower distal section of the guidewire. The combination, well understood in the art, provides the distal tip of the wire with a greater flexibility to steer through the vasculature of a patient, while the filament wire provides added strength and radiopacity. The filament wire can also be used as a fastening point for the attachment of an atraumatic tip. Examples of guidewires using the combination of transition areas and filament wires are described in Colon et al., (U.S. Pat. No. 5,402,799) and Ashby et al., (U.S. Pat. No. 5,622,184). Others have modified the basic design by using other materials, such as Johanson et al., (U.S. Pat. No. 5,596,996). However all of the prior art to date has used guidewires for essentially the same purpose, to navigate the anatomy of a patient and direct a catheter to a particular site within a body lumen. The medical procedure to be carried out is then conducted by the catheter. There are specialized guidewires which have been developed which attempt to do the job of a catheter using a modified guidewire. Two examples are guidewires with imaging and non-imaging sensors.
However there remains a need for a guidewire which can steer a catheter more particularly to a precise position within the vascular system of the patient. More particularly it would be beneficial to be able to manufacture a guidewire able to direct a catheter to a particular side of a lumen in the event a physician wishes to treat one side of a body lumen and not another, or be able to direct a catheter to precise locations of a body lumen. Straight guidewires are unable to perform this feat, however a novel guidewire has been disclosed in co-pending application Ser. No. 08/966,001 which is capable of steering catheters to a particular side of a body lumen. Furthermore, a method of determining the proper sizing of a medical device is desirable. At least some of these objectives will be met by the embodiments of the present invention described below.
The present invention relates to a guidewire for precise location of a medical device in a body lumen. A method for matching a guidewire of the present invention to a catheter is also provided. The guidewire of the present invention possesses a proximal end and a distal end with a compressible guide section comprising a plurality of helical winds located substantially at the distal end. Each helical wind of the guide section is capable of exerting an outward radial force when the guide section is compressed. The outward radial force is designed to exceed a catheter resistance force (Fc). The outward radial force the guide section exerts on the catheter (FGS) is produced by the portion of the helical wind that is held off a lumen wall. The portion held off of the lumen wall is the length of the guide section between the distal tip of a catheter tracking over it, and the point at which the guide section makes contact with the lumen distal to the catheter distal tip.
The guide section may be made with helical winds, such as a regular circular coil, or a near helical series of shapes, such as a polygon having a no sharp corners that would interfere with a medical device tracking over it, or pose a health risk to a patient. The outward radial force per unit length of the entire guide section is generally less than 4 pounds per centimeter of unconstrained length, and the radial force for any portion used for precision location is less than 2 pounds. The force for precision location is preferably less than one half pound.
The compressible guide section is made of a shape memory material, such as a metal alloy like nickel-titanium. Other materials may be used including ceramic composites or polymers provided the elastic and super-elastic strain of these materials is not exceeded during the actual use of the guidewire. The guide section may also be made from another low strain metal using a shape memory cladding. The guide section has sufficient outward radial force to overcome the inherent resistance force of either a standard over the wire catheter, or a rapid exchange (RX) catheter.
One alternative embodiment of the present invention is a guidewire having a proximal end, a distal end, and at least one displacement arm attached to the distal end. The displacement arm exerts an outward radial force when compressed. The displacement arm comprises a wire made of a shape memory alloy and operates similar to a single arc helical wind. The displacement arm anchors in the body lumen and deflects the guidewire tip into a lumen wall. Radiopaque markers provide a means for precision location of the wire in operation. The guidewire of this embodiment may have multiple displacement arms. When multiple displacement arms are used, the displacement arms preferably all have the same directional bias.
Another embodiment of the present invention comprises guidewire having a proximal end, a distal end and a lumen extending at least partially through said distal end. A filament wire is fixed to the interior distal tip of the guidewire. The filament wire is made from a shape memory material with a plurality of preformed curves. The guide section further comprises a plurality of apertures near the distal end of the guidewire. The preformed curves protrude through the apertures in the wire and act as spring detents for pushing the distal tip of the guidewire into the lumen wall. When a catheter tracks over the guide section and across the apertures, the filament wire compresses and lays completely within the filament wire lumen of the guide section. The spring detents are designed to xe2x80x9cstraddlexe2x80x9d the length of a rapid exchange catheter tracking over the distal end of the guidewire. By forming spring detents on either side of a rapid exchange catheter, the guide section of the guidewire is forced to abut the lumen wall.
A portable force resistance meter is also disclosed for determining a catheter""s force resistance value. The force resistance meter comprises an aperture for receiving a catheter distal tip, a deflection lever for moving said catheter distal end a quantifiable distance, a load cell linked to said deflection lever, a microprocessor and a display unit. The microprocessor may be programmed to display an appropriate matching guidewire for the catheter tested.
A method for determining the outward radial force a compressible guide section exerts can be determined following the steps of: incrementally axially displacing the guide section using a force-displacement measuring device, recording the axial force and axial and radial displacement at each increment, calculating the outward radial force from these measured values.
A method of determining a catheter resistance value is also described wherein the steps are selecting the length of the distal tip of the catheter to be deflected, performing a cantilever beam test over the chosen length, and calculating the force resistance value from the catheter stiffness measurements.
Finally, a method for matching a guidewire with a compressible guide section to a catheter for precision catheter positioning is disclosed. The method requires the steps of determining the desired lumen diameter to be treated, selecting a catheter and determining the catheter resistance force, and choosing a guidewire having a guide section with an outward radial force sufficient to deflect the catheter to the lumen wall. The relationship between a catheter and a guidewire are easily understood when employing either a graphing model of the forces, or an instrument such as a portable force meter with a programmed display.